Counterfeiting costs the world economy hundreds of billions of dollars per year. Aside from monetary losses, counterfeiting also represents a troublesome source of safety issues and health hazards due to the fact that counterfeited products rarely respect manufacturing standards. As a result, governments and institutions have, until now, maintained a suitable lead by wielding technology to their advantage. Consequently, various optical security features such as holograms, interference security image structures (ISIS), watermarks and micro printing have been in circulation for many years. Historically, currency has always been an early adopter of these technologies.
Interestingly, polymer banknotes are gaining in popularity; the most recent example being their adoption by the Bank of Canada and incremental integration into the Canadian market. These banknotes are not only more durable, they also offer the opportunity of creating optical windows, that is, regions of complete transparency. Thus, devices which were typically used in reflection can now be implemented in transmission and used in innovative ways. Incidentally, hybrid paper/polymer banknotes are also in development which will offer similar opportunities. Hence, the presently proposed feature takes advantage of this change in trend.
Interference-based security devices have been in circulation for more than two decades. Their iridescent properties, that is, their change in color as a function of the observation angle, have indeed been highly effective in inhibiting counterfeiting as well as accepted by the general public. Unfortunately, iridescent consumer products such as color shifting co-extruded polymer wrapping foil have now become readily available, and may render basic interference-based devices obsolete.
Classically, devices used in transmission are based on all-dielectric filters which are in first approximation absorption free in the region of interest i.e. the visible spectrum. On the other hand, although one can use such devices in transmission, their color in reflection and transmission is side-independent. The other alternative, which requires a lower amount of layers and thus is also the most popular, is metal-dielectric filters, but their use in a transmission mode is limited by their opaque metallic mirror. In addition, although the side-dependent reflection phenomenon which it is possible to generate using these structures is also known, to our knowledge it has yet to be exploited in optical security.
Metal-dielectric structures currently in use are typically based on the following three-layer system: an opaque metallic reflector, a dielectric spacer and a thin partially absorbing film. Such Fabry-Perot-like structures provide very high color saturation with a minimum of layers due to a combination of interference and of selective absorption. However, these structures are usually poorly adapted for uses where it is desirable to have a transmission mode and are limited to optical effects on a single side.
Also, metal-dielectric structure displaying angular color shift are disclosed in U.S. Pat. No. 8,064,632 to Baloukas et al. however the structures disclosed therein contain a large number of dielectric material layers making them more expensive to produce.
In light of the above there is a need for improved interference security image structures.